Field of the Invention
The present invention relates to a multipole lens and also to a charged particle beam system.
Description of Related Art
In recent years, there have been known charged particle beam systems such as a transmission electron microscope (TEM) and a scanning electron microscope (SEM) each equipped with an aberration corrector for correcting aberrations. Aberration correctors operate to correct aberrations by applying a magnetic field, an electric field, or a superimposition of these fields to a passage for a charged particle beam (such as an electron beam) by the use of multipole lenses (see, for example, JP-A-2005-302405).
For example, octopole fields are used for corrections of four-fold astigmatism and spherical aberration. In some cases, a low-order multipole field is superimposed on an octopole field to reduce the effects of axial deviation. In the case of a dodecapole lens, X- and Y-components of a quadrupole field and X- and Y-components of an octopole field can be created, for example, by applying magnetomotive forces to magnetic polepieces at ratios given in the table shown in FIG. 4. If the X-component of the quadrupole field is rotated through 45 degrees, the Y-component of the quadrupole field is obtained. If the X-component of the octopole field is rotated through 22.5 degrees, the Y-component of the octopole field results. The magnetomotive forces applied to the magnetic polepieces are determined by the numbers of turns of the coils wound on the magnetic polepieces and by currents flowing through the coils.
FIG. 5 schematically shows one conventional multipole lens, generally indicated by reference numeral 101A.
The multipole lens 101A is a dodecapole lens as shown in FIG. 5, and is configured including twelve magnetic polepieces 110-1 to 110-12, twelve coils 120, twelve power supplies 130-1 to 130-12, and a yoke 140. In the multipole lens 101A, the ratios of magnetomotive forces are controlled by the power supplies 130-1 to 130-12. Each of the coils 120 with the same shape and with the same number of turns is mounted to a respective one of the magnetic polepieces 110-1 to 110-12. Electric currents are supplied to these coils 120 from the independent power supplies 130-1 to 130-12, respectively. When one wants to superimpose the four kinds of fields consisting of the X- and Y-components of the quadrupole field and the X- and Y-components of the octopole field and to excite the coils and magnetic polepieces, the sum of magnetomotive forces to be impressed on the magnetic polepieces 110-1 to 110-12 is calculated using the table of FIG. 4. The amounts of currents passed through the coils 120 are found from the numbers of turns of the coils 120, and the outputs from the power supplies 130-1 to 130-12 are controlled.
FIG. 6 schematically shows another conventional multipole lens, generally indicated by reference numeral 101B.
As shown in FIG. 6, this multipole lens 101B is configured including twelve magnetic polepieces 110-1 to 110-12 (hereinafter may be collectively referred to as the magnetic polepieces 110), forty-eight coils 120, four power supplies 130-1 to 130-4 (hereinafter may be collectively referred to as the power supplies 130), and a yoke 140. In the multipole lens 101B, the ratios of the magnetomotive forces are controlled by the numbers of turns of the coils 120. The coils 120 having the numbers of turns which are proportional to the ratios of the magnetomotive forces listed in the table of FIG. 4 are mounted to the magnetic polepieces 110, respectively. These coils 120 are connected with their respective power supplies 130-1 to 130-4. As a result, the polepieces 110 are excited at the ratios of the magnetomotive forces creating the four kinds of fields, i.e., the X- and Y-components of the quadrupole field and the X- and Y-components of the octopole field, irrespective of the output currents from the power supplies 130.
If the coils 120 are controlled independently in the same way as for the multipole lens 101A shown in FIG. 5, various kinds of multipole fields can be generated. Furthermore, the number of the coils 120 can be suppressed to a minimum. However, in order to control the coils 120 independently, it is necessary to provide as many power supplies 130 as there are magnetic polepieces 110. Furthermore, variations in the currents flowing through the coils 120 give rise to deflecting fields and so the multipole lens is subject to the effects of noise.
The number of the power supplies 130 making up the multipole lens 101E shown in FIG. 6 is equal to a minimum number (four) of power supplies 130 producing the four kinds of fields. Furthermore, in the multipole lens 101B, the fields generated by the currents flowing through the coils 120 fundamentally contain no deflecting fields and, therefore, this lens is less subject to the effects of noise, it being noted that deflecting fields are generated only by axial deviations. However, the number of the coils 120 needed is equal to the number of fields multiplied by the number of magnetic polepieces. This greatly complicates wiring and terminal treatment. In addition, the ratios of the magnetomotive forces applied to the magnetic polepieces 110 must be controlled by the numbers of the turns of the coils 120, thus increasing the number of kinds of the required coils 120.